1. Field of the Invention
The invention relates to an explosive primer unit adapted to be initiated instantaneously by low-energy detonating cord (LEDC).
2. Description of the Prior Art
Blasting operations in which a cap-insensitive explosive is to be initiated non-electrically in a borehole usually are carried out with a cap-sensitive high-energy primer together with a high-energy detonating cord to initiate the primer when instantaneous initiation in the hole is required. The cord normally is threaded through a cord tunnel in the primer, a knot made in the cord to secure the primer on the cord, and the primer lowered into the hole by means of the cord. Multiple primers may be threaded onto a single length of cord and positioned at different depths in the hole.
High-energy detonating cords have an explosive core loading greater than 2 grams per meter, the cords most commonly used for the instantaneous initiation of primers having core loadings of 10 grams per meter. Cords of this size are disadvantageous, however, not only on the basis of cost, but also because the high noise levels associated with their use is objectionable in many locations. The use of high-energy detonating cords to initiate multiple primers instantaneously in a decked hole at delay intervals provided at the surface of the hole also is difficult to manage because, when two or more high-energy cords are used as downlines in such a situation, the detonation of one of the cords may initiate other cords instantaneously, thus overriding the surface delay timing imposed on the other downlines. Multiple downlines of high-energy detonating cord may be used successfully with surface delays only in holes of sufficiently large diameter and provided that extreme care is taken to keep the cords sufficiently separated from one another. Also, such decked holes should be initiated bottom deck first and upper decks consecutively in sequence.
Low-energy detonating cord (LEDC), having an explosive core loading of only about 0.02 to 2 grams per linear meter of cord length, overcomes the problems of noise, high brisance and initiating each other associated with the above-described high-energy cord. LEDC downlines heretofore have been employed in conjunction with high-energy primer or booster units containing a percussion-actuated detonator, e.g., in the delay booster assembly described in U.S. Pat. No. 3,709,149, issued Jan. 9, 1973, to H. E. Driscoll. Co-pending, co-assigned U.S. patent application Ser. No. 714,505, filed Mar. 25, 1985, by M. E. Yunan, describes a primer assembly which includes a percussion-actuated detonator seated in a cavity in a high-energy primer, and an explosive coupler for explosively coupling the detonator to LEDC which is to be threaded through a perforation in the primer. This assembly, when incorporating a delay detonator, can be used in an in-hole delay system for the delayed initiation of deck-loaded explosive charges with high-energy delay primers strung on a single LEDC downline. However, for the instantaneous initiation of a primer with an LEDC downline, a less complex primer assembly which still affords the reliable initiation of the primer explosive by the low-energy cord would be desirable.